Fix friction issues at low frame rates or repeated physics steps.

- Add `prevFrame` to hold previous frame positions (`prev` now holds
  previous step positions explicitly).
- Reset `prev` per step, fixing physics that relies on it.
- Remove `_reset` in favour of `moves`, which was all it ever checked.
  Ironically, `_reset` never reset itself.
- Remove some `postUpdate` property setting which is now unnecessary.
  The `prev` values now update just before they're used, so if you check
  them outside the physics step, they reflect the previous step.
- Reset `World#stepsLastFrame` per frame, so it won't get stuck on.

Author: BenjaminDRichards

src/physics/arcade/Body.js

@@ -159,6 +159,15 @@ var Body = new Class({
          */
         this.prev = new Vector2(gameObject.x, gameObject.y);
 
+        /**
+         * The position of this Body during the previous frame.
+         * 
+         * @name Phaser.Physics.Arcade.Body#prevFrame
+         * @type {Phaser.Math.Vector2}
+         * @author Benjamin D. Richards <benjamindrichards@gmail.com>
+         */
+        this.prevFrame = new Vector2(gameObject.x, gameObject.y);
+
         /**
          * Whether this Body's `rotation` is affected by its angular acceleration and angular velocity.
          *
@@ -696,17 +705,6 @@ var Body = new Class({
          */
         this.physicsType = CONST.DYNAMIC_BODY;
 
-        /**
-         * Whether the Body's position needs updating from its Game Object.
-         *
-         * @name Phaser.Physics.Arcade.Body#_reset
-         * @type {boolean}
-         * @private
-         * @default true
-         * @since 3.0.0
-         */
-        this._reset = true;
-
         /**
          * Cached horizontal scale of the Body's Game Object.
          *
@@ -909,10 +907,12 @@ var Body = new Class({
 
         this.preRotation = this.rotation;
 
-        if (this._reset)
+        if (this.moves)
         {
             this.prev.x = this.position.x;
             this.prev.y = this.position.y;
+            this.prevFrame.x = this.position.x;
+            this.prevFrame.y = this.position.y;
         }
 
         if (willStep)
@@ -936,6 +936,9 @@ var Body = new Class({
      */
     update: function (delta)
     {
+        this.prev.x = this.position.x;
+        this.prev.y = this.position.y;
+
         if (this.moves)
         {
             this.world.updateMotion(this, delta);
@@ -975,8 +978,8 @@ var Body = new Class({
      */
     postUpdate: function ()
     {
-        var dx = this.position.x - this.prev.x;
-        var dy = this.position.y - this.prev.y;
+        var dx = this.position.x - this.prevFrame.x;
+        var dy = this.position.y - this.prevFrame.y;
 
         if (this.moves)
         {
@@ -1009,8 +1012,6 @@ var Body = new Class({
 
             this.gameObject.x += dx;
             this.gameObject.y += dy;
-
-            this._reset = true;
         }
 
         if (dx < 0)
@@ -1031,16 +1032,10 @@ var Body = new Class({
             this.facing = CONST.FACING_DOWN;
         }
 
-        this._dx = dx;
-        this._dy = dy;
-
         if (this.allowRotation)
         {
             this.gameObject.angle += this.deltaZ();
         }
-
-        this.prev.x = this.position.x;
-        this.prev.y = this.position.y;
     },
 
     /**

src/physics/arcade/World.js

@@ -1046,6 +1046,7 @@ var World = new Class({
         //  We don't need to postUpdate if there wasn't a step this frame
         if (this.stepsLastFrame)
         {
+            this.stepsLastFrame = 0;
             for (i = 0; i < len; i++)
             {
                 body = bodies[i];